1. Field of the Invention
The present invention relates to a fuel injection pump for an internal combustion engine, more particularly, it relates to a fuel injection pump which supplies highly pressurized fuel under the control of a piezoelectric actuator.
2. Description of the Related Art
In a fuel injection pump of an internal combustion engine, especially a diesel engine, the fuel injection pump is preferably electronically controlled, to ensure that a precise fuel injection amount is obtained. In this connection, a pump in which an electromagnetic valve is provided for electronically controlling the fuel injection amount is well known. However, since diesel engines are now designed to be driven at high speeds, and because the number of cylinders provided has increased, a control device having a higher responsiveness than an electromagnetic valve is needed. Accordingly, a fuel injection pump provided with a piezoelectric actuator, which has a high responsiveness, has been proposed in a prior art. In such a fuel injection pump, a fuel injection period and amount are controlled by causing the piezoelectric actuator to expand and contract, thus varying a pressure in a control chamber. This variation in pressure causes a change in the position of a valve provided between a high pressure passage and a low pressure passage, and communication with between these passages is allowed or prevented according to the position of the valve.
Namely, the fuel injection pump is provided with a plunger slidably housed in a cylinder bore to define a high pressure chamber therein; the plunger moving backward to expand the high pressure chamber to cause fuel in a low pressure chamber to flow into the high pressure chamber, and then moving forward to compress fuel in the high pressure chamber and discharge the fuel outside of the fuel injection pump, and to cause undischarged surplus fuel to overflow from the high pressure chamber through an overflow passage.
However, in such a conventional fuel injection pump, the allowance and prevention of communication between the high and low pressure passages causes variations in the pressures acting on the valve, destabilizing the movement of the valve, and thus reducing the maximum fuel amount supplied.